Weight loss, in the context of medicine, health or physical fitness, is a reduction of the total body mass, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue. Therapeutic weight loss, in individuals who are overweight or obese, can decrease the likelihood of developing diseases such as type 2 diabetes, heart disease, high blood pressure, osteoarthritis, and certain types of cancer.  Attention to diet in particular can be beneficial in reducing the impact of diabetes and other health risks of an overweight or obese individual.

Weight loss occurs when an individual is in a state of negative thermodynamic flux: when the body is exerting more energy (i.e. in work and metabolism) than it is consuming (i.e. from food or other nutritional supplements), it will use stored reserves from fat or muscle, gradually leading to weight loss. It is not uncommon for some people who are currently at their ideal body weight to seek additional weight loss in order to improve athletic performance, and/or meet required weight classification for participation in a sport. However, others may be driven by achieving a more attractive body image.

There are many diet plans and recipes that can be helpful for weight loss. While some are classified as unhealthy and potentially harmful to one's general health, others are recommended by specialists. Diet plans are generally designed according to the recommended caloric intake but it is important to note that the most successful diets are those that simultaneously promote physical activity.